A wireless communication system has been developed in the direction of supporting a higher data transmission rate so as to meet wireless data traffic demands which have continuously increased. A conventional wireless communication system sought to develop a technology that mainly improves a spectral efficiency to increase the data transmission rate. However, due to increased demand for smart phones and tablet PCs and an explosive increase in application programs which require a large amount of traffic based on the increased demand, demand for data traffic has accelerated rapidly. Accordingly, it is difficult to meet the heavy increase in wireless data traffic demand through a frequency efficiency improvement technology alone.
One method to solve the above problem is to use a very wide frequency band. In a frequency band below 10 GHz which is used for a convention mobile communication cellular system, it is very difficult to secure a wide frequency band. Accordingly, securing a broadband frequency in a higher frequency band is required. However, as a transmission frequency for wireless communication becomes higher, propagation path loss increases. Therefore, a distance of arrival becomes shorter, which results in a coverage decrease. One of the main technologies to alleviate the propagation path loss and increase the distance of arrival in order to solve the above problem is a beamforming technique.
Beamforming may be divided into transmission beamforming which is performed by the transmitting side and reception beamforming which is performed by the receiving side. Transmission beamforming generally uses a plurality of antennas and concentrates signals transmitted from respective antennas in a particular direction (that is, space), so as to increase directivity. A set of a plurality of antennas is referred to as an array antenna, and an antenna included in the array antenna is referred to as an antenna element or an array element. The antenna array may be configured in various types such as a linear array and a planar array. When the transmission beamforming is used, the distance of arrival can be increased through a signal directivity increase, and the signal is not transmitted in any direction other than the corresponding direction. As a result, interference influencing another user can be significantly reduced.
The receiving side may perform the reception beamforming by using a reception array antenna. The reception beamforming concentrates received radio waves to be directed in a particular direction, increases sensitivity of signals received from the particular direction, and excludes signals received from another direction, so as to block interference signals.
In order to secure the wide frequency band, a super high frequency, that is, a millimeter (mm) wave system is introduced. As the transmission frequency becomes higher, the wavelength of a radio wave becomes shorter. Accordingly, when antennas are configured at half wavelength intervals, an array antenna may be formed by a larger number of antennas within the same area. That is, a communication system operating at an ultra high frequency band is well positioned to apply the beamforming technique because the communication system can acquire a relatively higher antenna gain in comparison with using the beamforming technique at a low frequency band.
Through the use of the beamforming technique, a performance index such as a Signal to Noise Ratio (SNR) can be optimized by maximizing a beamforming gain, but a diversity gain cannot be obtained since a multipath propagation is reduced. Further, performance sensitivity for the beamforming may be generated due to beam information mismatching according to a delay until an actual allocation after mobility or a channel condition of a Mobile Station (MS) and beam are measured/selected.
Accordingly, the conventional research was limited to determining beamforming weight coefficients for optimizing a performance index such as a received SNR by maximizing a beamforming gain when applying the beamforming.